1. Field of the Invention
The present invention relates to a battery state monitoring circuit and a battery device, and more particularly, to a technology of preventing an overcurrent from flowing through a battery or a device connected to the battery.
2. Description of the Related Art
FIG. 3 illustrates a circuit diagram of a related-art battery device. The related-art battery device includes a secondary battery 1, a charge/discharge control switch 2, a charge/discharge protection circuit 3, and external terminals 4 and 5. The charge/discharge protection circuit 3 includes a control circuit 22, a pull-down circuit 24, and an overcurrent detection circuit 35.
Next, an operation of the related-art battery device is described.
A voltage of the overcurrent detection terminal 14 is determined based on a resistive component of the charge/discharge control switch 2 and a current flowing through a load connected between the external terminal 4 and the external terminal 5. When abnormality occurs in the load, such as a short-circuit between the external terminal 4 and the external terminal 5, the load current is increased to raise the voltage of the overcurrent detection terminal 14.
When the voltage of the overcurrent detection terminal 14 exceeds an overcurrent detection voltage of the overcurrent detection circuit 35, the overcurrent detection circuit 35 outputs a detection signal. When receiving the detection signal, the control circuit 22 controls the charge/discharge control switch 2 so as to interrupt a discharge current. This state is referred to as “discharge overcurrent state”. On the other hand, when the voltage of the overcurrent detection terminal 14 falls below the overcurrent detection voltage of the overcurrent detection circuit 35, the overcurrent detection circuit 35 outputs a release signal to release the discharge overcurrent state (see, for example, Japanese Patent Application Laid-open No. 2006-101696).
In the discharge overcurrent state, the control circuit 22 turns on the pull-down circuit 24 to pull down the overcurrent detection terminal 14. Specifically, when the load returns to a normal state, the voltage of the overcurrent detection terminal 14 is decreased. Then, when the voltage of the overcurrent detection terminal 14 falls below the overcurrent detection voltage, the overcurrent detection circuit 35 releases the discharge overcurrent state. When receiving the release signal of the current detection circuit 35, the control circuit 22 controls the charge/discharge control switch 2 to restart the discharge. In this case, load impedance at which the discharge is restarted is referred to as “overcurrent release impedance”.
The pull-down circuit 24 is configured to be turned on when the load enters an abnormal state, and hence the discharge current flows via the pull-down circuit 24 in this case. Thus, the pull-down circuit 24 is set so as to have large impedance.
Specifically, the voltage of the overcurrent detection terminal 14 does not fall below the overcurrent detection voltage unless the load impedance becomes larger than the impedance of the pull-down circuit 24, and hence the discharge may not be restarted when the load impedance is not so large.